Electronic devices, such as switching regulators often generate electromagnetic interference which can be detrimental to the operation of electronic circuits in which such devices are employed. Switching regulators translate all input voltage at one level to an output voltage at another. Energy from an input voltage source is stored in an inductor or capacitor and then transferred, or switched, to the output. Energy transfer is repeated at a rate determined by the clock source of the regulator. The switching action generates interference, the energy of which is concentrated at frequencies which are harmonics of the clock source.
The output of a switching regulator is a DC level signal plus interference created by clock-controlled switching. Different switching regulators require different clock frequencies. Consequently, the interference from a switching regulator differs from design to design. Techniques to reduce interference should be effective for clocks of both slow and fast clocking frequencies. If the clock frequency of a regulator is dynamically varied, it is possible to spread the interference energy over a range of frequencies, reducing the energy at any one frequency. This technique is known as spread spectrum frequency modulation.
Methods to implement spread spectrum frequency modulation have included modulation with periodic waveforms such as triangular or sinusoidal waveforms, and random modulation. However, each of these techniques has drawbacks.
The shape of a modulating waveform will affect the shape of the interference spectrum. FIG. 1 illustrates the output of a typical switching regulator with periodic sinusoidal frequency modulation of the clock source. The upper trace 102 is the modulating signal. The lower trace 104 is the output of the regulator. The lower trace 104 appears as an amplitude variation of the switching induced interference. Even though the interference energy near the clocking frequency has been “spread,” a low frequency large amplitude component has been added to the spectrum. This “amplitude modulation” could easily corrupt the operation of electronic circuits, a result which nullifies the benefits of the sinusoidal modulation.
Random modulation provides an improvement over the use of a periodic waveform. If the clock source of a switching regulator is randomly varying, using, for example, a technique termed “frequency hopping,” then the resulting amplitude modulation of the interference will be random. Random amplitude variations are indistinguishable from noise. However, the difficulty with random modulation lies in the generation of an optimal random signal. A random signal should be provided that produces reduction of EMI at both fast and slow hopping rates within the ability of practical regulators to track them. Lowpass filtering is employed for this purpose, without which the output will tend to exhibit ripple. Too much filtering, however, will negate the benefit of random modulation.
There is a need for a product which generates a wide range of clock frequencies to cover many switching regulator applications. At any clock frequency, the product should exhibit random frequency modulation which is fast enough for good EMI reduction, yet not too fast for switching regulators to track.